Anal. Chem. 1987, 5 9 , 1685-1691
thod of producing MS/MS spectra. These spectra show a strong dependence on the scan parameters used (and by implication, the energy deposition) so that care must be exercised in choosing and maintaining particular instrumental parameters. This is a consequence of the complex ion motion in the trap and hence the distribution of velocities in the ion population. The possibility of studying ion/molecule reactions adds further information and complexity. However, ion trap MS/MS spectra can be made highly reproducible, and the three principal factors that determine the appearance of these spectra, qL,supplementary ac voltage, and ac voltage duration are readily controlled. LITERATURE CITED (1) Tandem Mass Spectrometry; McLafferty, F. W., Ed.; Wiley: New York, 1983. (2) Harris, R. M.: Beynon. J. H. I n Gas Phase Ion Chemistry; Bowers. M. T., Ed.; Academic: New York, 1984; Vol. 3, Chapter 19, pp 100-127. (3) Kenttlmaa, H. I.; Wood, K.: Busch, K. L.; Cooks, R. G. Org. Mass Spectrom. 1983, 18, 561-567. (4) Beynon, J. H.; Caprioli, R. M.; Baitinger, W. E.; Amy, J. W. Org. Mass Spectrom. 1970, 3, 455. (5) Kinter, M. T.; Bursey, M. M. J . Am. Chem. SOC. 1988, 108, 1797. (6) Fischer, E. Z . Phys. 1959, 156, 1-26. (7) Fischer, E. Doctoral Dissertatlon, University of Bonn, 1958. (8) Stafford, G. C.. Jr.; Kelley, P. E.; Syka, J. E. P.; Reynolds, W. E.; Todd, J. F. J. Int. J . Mass Spectrom. Ion Processes 1984, 6 0 , 85-98. (9) Wineiand, D. J.; Itano, W. M. Adv. At. Mol. Phys. 1983, 19, 135. (10) Comisarow. M. 6.: Marshall, A. G. Chem. Phvs. Lett. 1974. 25. 282-283. (I 1) Wanczek, K. P. Int . J . Mass Spectrom. Ion Processes 1984, 6 0 , 11-80. (12) Lande, D. A,. Jr.; Johlman, C. L.; Brown, R. S.;Weil, D. A,; Wilkins, C L. Mass Spectrom Rev. 1986, 5 , 107-166.
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(13) Yost, R. A.; Enke, C. G. Anal. Chem. 1979, 5 1 , 1251A-1264A. (14) Paul, W.; Reinhard, H. P.; von Zahn, V. Z . Phys. 1958, 152, 143-182. (15) Lawson, G.; Todd, J. F. J.; Bonner, R. F. Dyn. Mass Spectrom. 1976, 4 , 39-81. (16) Dawson, P. H. Quadrupole Mass Spectrometry; Elsevier: New York, 1976; Chapters 2-3. (17) Kelly, P. E.; Stafford, G. C.. Jr.; Syka, J. E. P.; Reynolds, W. E.; Louris, J. N.; Amy, J. W.! Todd, J. F. J. 33rd Annual Conference of Mass Spectrometry and Allied Topics, San Diego, CA, 1985; p 707. (18) Louris, J. N.; Brodbelt, J.; Cooks, R. G.; Stafford, G. C., Jr.; Syka, J. E. P. 34th Annual Conference of Mass Spectrometry and Allied Topics, Cincinnati, OH, 1986; p 685. (19) Kelley, P. E.; Syka, J. E. P.; Ceja, P. C.; Stafford, G. C.. Jr.; Louris, J. N.; Grutzmacher, H. F.; Kuck. D.; Todd, J. F. J. 34th Annual Conference of Mass Spectrometry and Allied Topics, Cincinnati, OH, 1986: p 963. (20) Griffiths, I.W.; Harris, F. M.; Mukhtar, E. S.;Beynon, J. H. I n t . J . Mass Spectrom. Ion Phys. 1981, 4 1 , 83. (21) Chen, J. H.; Hayes, J. D.; Dunbar, R. C. J . Phys. Chem. 1984, 8 8 , 4759-4764. (22) McLuckey, S. A.; Sallans, L.;Cody, R. 6.; Burnier, R. C.; Verma, S.; Freiser, B. S.;Cooks, R. G. Int. J . Mass Spectrom. Ion Phys. 1982, 4 4 , 215. (23) McLuckey, S.A.; Ouwerkerk, C. E. D.; Boerboom, A. J. H.; Kistemaker, P. G. Int. J . Mass Spectrom. Ion Phys. 1984, 5 9 , 85. (24) Nacson, S.; Harrison, A. G. Int. J . Mass Spectrom. Ion Processes 1985, 63, 325. (25) Wysocki, V. H.; Kenttamaa, H. I.; Cooks, R . G. Int. J . Mass Specfrom. Ion Processes in press. (26) Kelley, P. E.; Stafford, G. C.; Syka, J. E. P.; Reynolds, W. E.; Louris, J. N.;Todd, J. F. J. Adv. Mass Spectrom. 1985, 10, 869-870. (27) Brodbelt, J. S.;Louris, J. N.; Cooks, R . G. Anal. Chem., in press. (28) Louris, J. N.; Brodbelt, J. S.;Cooks, R. G. Int. J . Mass Spectrom. Ion Processes, in press.
RECEIVED for review December 23, 1986. Accepted March 19, lgg7* This work was supported by the National %ence Foundation CHE 84-08728.
Focused, Rasterable, High-Energy Neutral Molecular Beam Probe for Secondary Ion Mass Spectrometry Anthony D. Appelhans, James E. Delmore,* and David A. Dah1
Idaho National Engineering Laboratory, EG&G Idaho, P.O. Box 1625, Idaho Falls, Idaho 83415
Beams of focused (-0.5 mm dlameter at 2.5 m), rasterable, 3-17 keV neutral SF, molecules produced via autoneutrallzatlon from the correspondlng anlon have been demonstrated to be effective for produclng secondary Ions from electrically insulating speclmens (e.g., Mylar, Teflon) without sample charging problems. Secondary ion currents measured with A ( lo5 counts/s equlvalent) for the quadrupole were major peaks when uslng a picoampere (neutral equivalent) level prlmary beam which was more than adequate to characterize the speclmens. No contamlnatlng effects from S or F were seen In the spectra. The results demonstrate the feaslblllty of using the autoneutrallrlng SF, fast neutral source for a variety of static secondary Ion mass spectrometry applications that require electrically neutral primary particles focused into a near-parallel small diameter beam (over distances of 18 cm to several meters). Appllcatlons could include the analysis of polymers, ceramics, and biospeclmens or the generation of Ions lnslde the magnetically conflned cell of an Fourler transform Ion cyclotron resonance mass spectrometer.
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Secondary ion mass spectrometry (SIMS) and fast atom bombardment mass spectrometry (FABMS) have found many distinct areas of application in recent years (1). A variety of
ion and neutral atom sources have been developed for these applications, ranging from micrometer-dimensioned ion beams capable of imaging to a variety of schemes for producing broad, unfocused beams of neutral atoms. However, one area that has seen little development is that of focused (as opposed to collimated), rasterable neutral atom/molecule beams. There are several areas of applicability for such a primary beam probe. The principal application of static SIMS/ FABMS has been the study of large, fragile molecules most of which are electrically nonconducting. A focused, rasterable, small diameter (millimeters and less) neutral primary beam capable of producing easily measurable secondary ions at low (nondamaging) fluence would simplify analysis of small areas (interfaces, boundary regions) on polymers, ceramics, and biological specimens. Extending this to spatial analysis/imaging requires a microfocused neutral beam that can be rastered over the sample. This type of imaging has recently been demonstrated by Eccles e t al. ( 2 ) ,although using an approach substantially different from the one taken in this study. In addition to the obvious applications of a focused neutral beam for SIMS there is another area where such a beam has significant potential application: sputtering of secondary ions from samples in the magnetically confined ion cell of Fourier transform ion cyclotron resonance (FTICR) mass spectrometers. The magnetic and electric fields of the FTICR cells make it difficult to inject a beam of charged particles into the
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ion trapping region. Although methods employing tandem quadrupole ion beam transmission systems have been used experimentally (3) to generate secondary ions in an ICR cell, it necessitates a complex apparatus and is plagued by sample charging problems (4)inherent to using ions as the primary sputtering beam. Current commercial FAB sources do not have sufficiently fine focusing for such applications and the high gas load imposed by charge-exchange neutralization cells (2) is unacceptable for the high vacuum requirements of FTICR. A pulsed, rasterable, near-parallel focused beam of high-energy neutral particles, unaffected by magnetic and electric fields and generated in a differentially pumped primary source a t a relatively low source pressure (- IO4 torr), should prove an ideal probe for producing secondary ions in FTICR systems. To produce such a beam, we have applied a unique neutralization mechanism: autoneutralization, the spontaneous ejection of an electron from an anion leaving a neutral molecule (5,6).The specific ion being used is the sulfur hexafluoride anion, a significant fraction of which autoneutralizes in 2-20 ps. Sulfur hexafluoride has a unique combination of properties: a very high electron capture cross section, which greatly simplifies the production of the gas phase anion, and a relatively low electron affinity (- 1 eV), which expedites the detachment of electrons from the gas phase anions that are in certain excited quantum states. Earlier studies in our laboratory (6) have shown that there are several autoneutralizing excited states, corresponding to different excited quantum states, and that about 3 5 4 0 % of the ions undergo the second step of the following sequence:
where I l k , the mean lifetime, is 10-20 ps. This makes it possible to focus and accelerate the SF6-ions prior to autoneutralization and still achieve adequate neutralization within a reasonable length flight tube ( 18 cm) to yield a usable neutral beam. The composite lifetime is well suited to accelerating the anions to energies ranging from 3 keV up to 25 keV (or more). Recent unpublished results from this laboratory have shown that the energy state distribution can be shifted toward the shorter lived states (-2 1s) by heating the gas, thus improving neutralization efficiency. The mass of the ejected electron is of the order of lo4 that of the ion, thus there is little translational energy transferred to the molecule during neutralization. This permits the molecules to maintain focus as neutralization occurs. There are both advantages and disadvantages to the autoneutralization approach. The advantages are the following: (1)the particles can be accelerated to much higher energies (eV/amu) than practical in conventional FAB sources with minimum loss of neutralization efficiency; (2) the autoneutralization mechanism does not defocus the beam; (3) the gas load on the sample chamber from the primary ion source is small (primary source pressure -lo4 torr); (4) high neutralization efficiency (-15% within an 18-cm and -40% within a 200-cm drift tube at 16 kV acceleration). The disadvantages are the following: (1) the requirement of completing all focusing either in or just after the acceleration lens (it is not possible to refocus the beam just prior to the sample as is often done with ion beams); (2) the length of the drift tube (neutralization region). Prior to the experiments reported here the performance of the seven-atom SF6molecule (vs. an atom) for the production of secondary ions was not known; thus it was unclear as to whether the large size of the primary particle would be an advantage or a disadvantage. Geometric coupling between the primary beam molecules and the surface molecules could conceivably play a role in desorbing secondary ions.
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IBEAM_"IEWtNG-
FARADAY CUP
E X B ELECTRON CAPTURE ION SOURCE
d O C U S I N O AND RASTERING ELECTRODES
SAMPLE PROBE
1 QUADRUPOLE\
Figure 1. Schematic diagram of the SF, neutral beam prototype. The flight tube length is oversized to permit special experiments to be performed.
The question of the performance of the seven-atom molecule in sputtering ions from specimens is the most important issue to resolve prior to committing to a major effort to develop this technology into an analytical tool. Previous work (7) has shown that the energy and momentum of the primary particles affect the secondary ion sputtering efficiency. I t also seems reasonable to expect that energy density (instantaneous energy deposited per unit surface area of the sample) would also play a role, particularly for sputtering large molecular ions. The kinetic energy of the SF6molecule is distributed among the seven atoms and the SF6 molecule is large (geometrically) compared to an atom. The SF6 molecule would need to be accelerated to a higher velocity than an atom to obtain an equivalent energy density on the sample matrix, however the energy density would be produced over a larger area compared to the atom. A typical SIMS or FAB source operates at about 5 kV, resulting in an energy of 122 eV/amu for Ar (typical) atoms. Thus a voltage of 18 kV would be necessary to obtain an equivalent energy/amu for SF6. Thus, with the previous demonstration (5,6) that the autoneutralization mechanism can be applied to produce a focused, small-diameter neutral beam that can be transported over meter distances, the objectives of the studies reported here were to (1) demonstrate the efficiency of fast (3-17 keV) neutral SF, molecules for desorbing secondary ions from a variety of nonconducting polymer surfaces and (2) do this without the use of any external surface charge neutralization. The results demonstrate the feasibility of developing this technique for the applications discussed.
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EXPERIMENTAL SECTION The experimental system consists of an ion source with a focusing and accelerating lens, a 2.5-m flight tube (longer than necessary for most applications), a multichannel plate array detector (MPAD) and viewing screen for beam display, a Faraday cup, sample introduction system, and a secondary ion quadrupole mass spectrometer. SF6- ions are produced in the source, and focused and accelerated prior to entering the flight tube through a differential pumping slit. Within the flight tube autoneutralization occurs producing the neutral beam. Those ions that have not neutralized by the end of the flight tube are electrostatically deflected away from the sample and into the Faraday cup, permitting dynamic monitoring of the beam stability. Both the source housing and sample/detector housing are pumped with 170 L/s turbomolecular pumps. Figure 1 shows a schematic diagram of the system. The electron capture cross section of SF6 is dependent upon the electron energy, requiring
